This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-38081 filed on Feb. 15, 2001 and Japanese Patent Application No. 2001-317587 filed on Oct. 16, 2001.
1. Field of the Invention
The present invention relates to a coolant temperature estimation system of an internal combustion engine, which renews an estimated coolant temperature value based on an operating state of the engine during engine operation.
2. Description of Related Art
For example, Japanese Unexamined Patent Publication No. 2000-220456 proposes a malfunction detection system for detecting malfunction of a thermostat of an engine cooling system. In the malfunction detection system, an amount of heat generated from an engine, which corresponds to an operating state of the engine (the amount of air filled in the respective cylinders), is converted to an increase in coolant temperature at predetermined time intervals during the engine operation. Then, the increase in the coolant temperature is added to a previously estimated coolant temperature value to obtain a currently estimated coolant temperature value. Abnormality of the thermostat is judged by determining whether a difference between the currently estimated coolant temperature value and an actual coolant temperature measured with a coolant temperature sensor is greater than an abnormality judgment threshold value.
Generally, an engine installed in a vehicle cuts fuel supply to stop fuel injection during a vehicle speed reducing period and also during a high engine speed period in order to improve fuel consumption during the vehicle speed reducing period and to prevent engine damage during the high engine speed period. During the fuel cutting operation, heat of combustion is not generated in the engine, and thus it is assumed that a coolant temperature is reduced through heat release from the coolant during the fuel cutting operation in the above system. As a result, in the above system, the estimated coolant temperature value is gradually reduced during the fuel cutting operation as a function of time elapsed since the initiation of the fuel cutting operation.
However, one recent experimental result indicates that the coolant temperature value continues to increase for a while even after the initiation of the fuel cutting operation. This is probably due to the following two reasons. Firstly, even during the fuel cutting operation, the intake air supplied to the respective cylinders of the engine is compressed therein, so that the heat of compression is generated in the respective cylinders. Secondly, the cylinder block temperature is higher than the coolant temperature during the engine operation, so that the heat accumulated in the cylinder block is conducted to the coolant to increase the coolant temperature during the fuel cutting operation.
Thus, if the estimated coolant temperature value is gradually reduced during the fuel cutting operation as a function time elapsed since the initiation of the fuel cutting operation, as in the case of the above system, a difference between the estimated coolant temperature value and the actual coolant temperature is increased during the fuel cutting operation. This causes deterioration of estimation accuracy of the coolant temperature. As a result, a normal thermostat could possibly be mistakenly judged as abnormal.
The present invention addresses the above disadvantages. Thus, it is an objective of the present invention to provide a coolant temperature estimation system of an internal combustion engine capable of minimizing deterioration of coolant temperature estimation accuracy induced by a fuel cutting operation and also capable of achieving more reliable coolant temperature estimation.
To achieve the objective of the present invention, there is provided a coolant temperature estimation system for estimating a temperature of coolant of an internal combustion engine. The system includes a coolant temperature estimating means and a coolant temperature estimation prohibiting means. The coolant temperature estimating means renews an estimated coolant temperature value based on an operating state of the internal combustion engine during operation of the internal combustion engine. The coolant temperature estimation prohibiting means prohibits renewal of the estimated coolant temperature value upon elapse of a predetermined time period since initiation of a fuel cutting operation until end of the fuel cutting operation.
To achieve the objective of the invention, the system can alternatively include an estimated coolant temperature correcting means in place of the coolant temperature estimation prohibiting means. The estimated coolant temperature correcting means corrects the estimated coolant temperature value in such a manner that an increase in the estimated coolant temperature value is progressively reduced as a function of time elapsed since the initiation of the fuel cutting operation during the fuel cutting operation.
Furthermore, to achieve the objective of the present invention, there may be alternatively provided a coolant temperature estimation system for estimating a temperature of coolant of an internal combustion engine. The system includes a coolant temperature estimating means. The coolant temperature estimating means renews an estimated coolant temperature value based on an operating state of the internal combustion engine during operation of the internal combustion engine. The coolant temperature estimating means includes a fuel cutting period coolant temperature estimating means, which computes the estimated coolant temperature value during a fuel cutting operation. The fuel cutting period coolant temperature estimating means is selected and used during the fuel cutting operation to renew the estimated coolant temperature value in such a manner that an increase in the estimated coolant temperature value during the fuel cutting operation is reduced in comparison to an increase in the estimated coolant temperature value during a normal operation.